Technical Field
The present description relates to techniques for predicting the orbit of a satellite used by a satellite signal receiver by acquiring satellite signals and navigation data, and calculating a position solution, including predicting the state or orbit of one or more satellites that will be used by a GNSS receiver, said prediction including using a model of the solar radiation pressure operating on said one or more satellite.
Various embodiments may apply e.g., in a GNSS receiver to achieve a faster position fix, in particular for GPS block IIR satellites or IIF block satellites.
Description of the Related Art
Autonomous assisted GNSS (Global Navigation Satellite System) technology works by predicting the state (including position and velocity) of the satellites that will be used by a satellite signal receiver such as a GNSS receiver to achieve the so-called fix, i.e., acquiring satellite signals and navigation data and calculating a position solution, so that it will not be necessary to wait until the broadcast ephemerides are fully downloaded from the sky or from the Internet. To predict the current state of the satellite one or more ephemerides are used, which have been previously, in particular in the recent past, downloaded, either from the sky or from the Internet.
The Time To First Fix (TTFF) performance of GNSS receiver can be improved, allowing it to achieve faster fix either if the receiver is not connected to the Internet at all, or if it is connected but the connection has a high latency or is not available at the moment of the fix.
Therefore, GNSS satellite orbit prediction is very useful to dramatically reduce the Time To First Fix of a GNSS receiver whether it is connected to the Internet or not.
GNSS satellite orbit prediction uses the models of the forces acting on the spacecraft represented by the satellite to predict the future orbit of the spacecraft itself.
The four main forces acting on GNSS satellites are Earth gravity, Sun gravity, Moon gravity and solar radiation pressure.
Among those forces, the solar radiation pressure is the only one that requires a model, which depends upon the spacecraft's type and mass to calculate the acceleration that it produces on the spacecraft itself.
For the above reason it is important to apply a reasonably accurate solar radiation pressure model for each typology of spacecraft. As said, autonomous assisted GNSS prediction requires the knowledge of the spacecraft's type and mass.
The navigation message transmitted by the GNSS satellite (which includes the broadcast ephemeris, from which it is possible to get information on the current state of the satellite—for instance position and velocity) does not contain any information about the satellite type.
Besides, satellites are periodically replaced, and the replacing spacecraft type could be different from the one being replaced. Even if replacement of satellites is a slow process, once it happens it could lead to a worse prediction of the spacecraft orbit and therefore to a deterioration of assisted GNSS performance.
Moreover, satellite mass tends to decrease as time passes, since fuel is spent for orbit correction maneuvers. Even if that is a slow process too, it could lead to a gradual deterioration of assisted GNSS performance.
Known solutions store the satellite type information statically in the receiver device.
Spacecraft replacement could lead to a worse prediction of the satellites orbit (due to the solar pressure model which could not fit the replacing spacecraft) and therefore to a deterioration of autonomous assisted GNSS performance based on that solution.
To avoid assisted GNSS performance deterioration with known solutions the satellite solar radiation pressure model sometimes needs to be updated from the outside (for example with a firmware update of the receiver).
Therefore, if the GNSS receiver is not updated, it could show assisted GNSS performance deterioration.
The subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, the recognition of one or more problems in the prior art discussed in the Background section and the subject matter associated therewith should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion in the Background section encompassing one or more recognized problems in the prior art should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.